It is sometimes necessary for an orthopedic surgeon to perform accurate and precise bone shaping. For example, precise cutting of the femur and tibia in the area of the knee is reguired to shape the bone ends to receive prostheses during knee joint replacement surgery. Such preparation involves resection of (removal of bone from) the anterior, posterior and distal femoral condyles and the proximal end of the tibia. The cuts must be precisely placed so that the resulting shaped bone structure very closely fits the intended prostheses and so that the tibia and femur are properly aligned after implantation of the prosthesis. Precise bone cuts are also reguired when an osteotomy is performed to realign an articular joint with its physiological mechanical axis. Such preparation involves resection of a wedge of bone adjacent to the articular surface, thereby realigning the transverse axis of the joint relative to the axis of the supporting bone. The cut surfaces are planar to enable complete surface contact of the surfaces when bones are repositioned and secured in place.
At one time, shaping cuts in the bones were made without benefit of cutting guides. Surgeons attempted to correct errors made during the cutting process by application of varying guantities of cement while securing a prostheses to the bone. New orthopedic techniques include the use of porous coated prostheses. The new prostheses promote bone ingrowth; they are used with minimal or no cement. Commonly, the cement is applied in a thin layer of uniform thickness. In that case, cutting errors can no longer be easily corrected and it becomes crucial that cuts be accurately made to within a few thousandths of an inch.
The necessity for precise and accurate bone cutting has led to the development of instruments specifically designed to guide the surgeon's saw blade during the bone cutting procedure. Examples of such instruments include those sold as the Howmedica.RTM. P.C.A..TM. Universal.TM. Total Knee Instrument System by Howmedica, Inc. Reference is made to the publication entitled "The PCA.TM. Primary Total Knee System", brochure No. ST2014, 1984, the teachings of which are incorporated by reference herein. Another example of instruments are those sold as the Whiteside Ortholoc.TM. Total Knee System by Dow Corning/Wright. Reference is made to the brochure entitled "Whiteside Ortholoc.TM. Total Knee System: Surgical Procedure", Form No. L095-02011, 1984, the teachings of which are incorporated by reference herein.
In one bone shaping technique, femoral cuts are made as follows: an intramedullary alignment rod is driven upwardly within the shaft of the femur for perhaps eight inches, the protruding end of the rod being used to locate precisely the cuts that are needed. Using various marking and measuring instruments, a saw guide is eventually mounted to the rod, the saw guide having slots in it through which the saw blade can extend during a sawing operation. In this manner, the distance between cuts and also the general plane of the cuts may be at least partially controlled. Unfortunately, the toothed end of the saw blade is somewhat thicker than is the body of the saw blade. To accommodate insertion of the saw blade, the slots are formed slightly wider than the thickness of the saw blade, affording the saw blade a substantial degree of freedom to wobble. The wobbling can cause cuts to be made having substantial errors in the angle of the plane of the cut. Whiteside U.S. Pat. No. 4,467,801 describes a cutting guide for a bone cutting saw blade having a slot such as the one described above. Petersen, U.S. Pat. No. 4,524,766, also describes a cutting guide for a bone cutting saw blade having slots in which a saw blade fits.
Another similar saw guide uses a flat guide surface (rather than a slot) to position and guide the saw blade. It is intended that the surgeon will maintain surface-to-surface contact between the saw blade and the planar guide surface during the cutting operation. It will be understood that bone properties vary widely; from very soft bone to very hard bone, including patches of soft bone within a hard bone structure. Hence, it is difficult to both maintain surface-to-surface contact between a saw blade and a planar guide surface and at the same time exert sufficient strength and force to appropriately cut the bone. For this reason, cuts that are made utilizing guides with flat guide surfaces sometimes are not properly positioned. Saw cutting guides having a flat guiding surface are described in the Howmedica surgical technique brochure No. ST2014, 1984 mentioned above.